1. Field of the Invention
The present invention relates to a camera, and more specifically, to a camera having an auto focus (AF) function.
2. Description of the Prior Art
A function called continuous AF has conventionally been known. Continuous AF is a function of repeating an AF operation such as focus detection and lens driving so that an in-focus condition is always obtained with respect to a subject moving along the optical axis.
On the other hand, while photometry and an AF operation are started by a halfway depression of a release button in conventional cameras, a camera has recently been proposed where photometry and an AF operation are started prior to the halfway depression of the release button in order to reduce time lag as much as possible. For example, a camera where photometry and an AF operation are started on it being sensed that a user is looking through the finder is proposed in Japanese laid-open Patent Application S64-42639.
In a camera of this type, since it is necessary to always obtain an in-focus condition during the whole time the user is looking through the finder, it is desirable to perform the above-mentioned continuous AF. In performing continuous AF under a condition where in-focus condition has been obtained, however, since variation in focus position which variation is caused as the subject moves is not so large, it is desirable to give priority to causing the user to be unconscious of an AF operation over the operation speed of the continuous AF. That is, a quiet continuous AF is desirable which causes the user to be unconscious of focusing.
On the contrary, if an AF operation is slowly performed on the user's looking through the finder, an amount of time is required from when the user starts to look through the finder to when an in-focus condition is obtained, which is inconvenient to the user. Moreover, it is required that focusing be immediately performed just before a release operation is performed.
As described above, concerning the cameras having the continuous AF function, a camera is demanded where lens driving control can be changed according to the situation.
On the other hand, continuous AF has another problem. Generally, focusing is performed with respect to a subject located in a predetermined area called a focus area in conventional cameras. With such an arrangement, when a main subject that the user intends to photograph is located outside of the focus area, an in-focus condition is obtained with respect to the background, not with respect to the main subject. Such a problem is also caused in performing a continuous AF operation. Particularly, in a case where the main subject moves out of the focus area just before a release operation, the moment the main subject moves out, in-focus condition is obtained with respect to the background in the continuous AF. As a result, a photograph is taken where the main subject is out of focus.